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My Friends Won't Let Me Play Zombies Anymore

Magstrike from hell, a preview of doom to come

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#26 Ambience 327

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Posted 20 March 2009 - 01:04 PM

Bob is definately correct. You're much better off wrapping it in several loose layers of denim. Heck, a few layers of loosely wrapped corrugated cardboard or bubble wrap would be far better than what you've suggested!
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#27 Doom

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Posted 20 March 2009 - 01:34 PM

Denim with a 2 liter bottle should work, but it's fairly ghetto. I've also seen people use chicken wire but I don't know much about chicken wire's effectiveness.

Consider an aluminum LPA tank. I bought one for my FANG 2 prototype and it was very well worth the time calling in and the money. The tank itself has a pressure rating of 240 psi and a burst pressure of 600 psi. It's also incredibly light. I won't say how much I paid because Catalina Cylinders doesn't usually sell single units, but I will say it's definitely cheaper than soda unless they've changed their prices or policies since February.

If anyone is interested in calling them up, I suggest being very honest but short about what you'll use it for. They sell primarily in bulk but are willing to consider single units if they like your project.

Edit: The threads on the Catalina LPA tanks are some odd custom thread. Catalina sells a fitting called the F1 that goes from their thread to 1/4" NPT. Also, their abilities are not limited to what they show online, so if you want something custom they might be able to do it or have something similar already.

Looks great! However, wouldn't the range decrease after every shot? Unlike the stock bladder, your replacement "bladder" maintains its volume as the overall air pressure decreases. With the stock bladder, the volume decreases to accommodate for the loss of air pressure which in turn maintains the air pressure in the bladder. I may be completely off on this, so if I am, just let me know.


That's one way to think about it. My conception of how bladders maintain relatively constant pressure is that the total force per unit area (pressure) they apply is relatively constant if you slice the bladder up into segments like rubber bands (they are all stretched the same length, so they apply the same force per unit area). Another way to think about it is that as the total applied force increases (from more of the bladder expanding), the surface area increases to compensate to keep the pressure constant.

Edited by Doom, 20 March 2009 - 02:12 PM.

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#28 diamondbacknf1626

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Posted 20 March 2009 - 03:32 PM

Bob, not trying to start a flame war here, but that was not out of "thin air." That was actually "borrowed" from a friend's project that he was working on. However, he was working with much less psi, so my suggestion may be entirely invalid. Just to be clear, I wasn't suggesting just wrapping strands of e-tape around the bottle, but cover the entire thing with a coil of e-tape, so that no bottle is showing. Again, I may very well be entirely wrong, but I'm curious as to why that would make it more dangerous.

By the way: I like the ideas with the denim and ducktape. They seem far more reliable than the e-tape cocoon
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#29 Pokechan

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Posted 20 March 2009 - 05:35 PM

On top of adding a PVC cover to the tank, put holes in the PVC. BIG holes. If that thing explodes inside it, and there is no exit, PVC shrapnel will kill. Trust me, I have had experience
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#30 Draconis

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Posted 20 March 2009 - 06:11 PM

Bob, not trying to start a flame war here, but that was not out of "thin air." That was actually "borrowed" from a friend's project that he was working on. However, he was working with much less psi, so my suggestion may be entirely invalid. Just to be clear, I wasn't suggesting just wrapping strands of e-tape around the bottle, but cover the entire thing with a coil of e-tape, so that no bottle is showing. Again, I may very well be entirely wrong, but I'm curious as to why that would make it more dangerous.



It may keep more pieces connected to each other, but would not add any strength to the system. I'm surprised at how few of you kids have made Piccolo **** bombs, as they are made by wrapping a firework with electrical tape and... No, I'm not going to tell you. Never mind.
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#31 Spartan 117

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Posted 20 March 2009 - 06:24 PM

Woah! A beast for sure! Just one question though. how well does it pressurise with that makeshift tank. I'm asking because the stock magstrike has a rubber bladder.
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#32 Mr BadWrench

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Posted 21 March 2009 - 05:28 AM

http://www.baitnhook...etyairhorn.aspx

they sell these.... they are rated to 100 psi... its just a pop bottle.
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#33 Bullshit Dragon

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Posted 23 March 2009 - 02:51 AM

I'd like to point out that Mythbusters got a 2 liter coke bottle up to 170psi before it burst.
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#34 slowguitarman

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Posted 23 March 2009 - 08:53 AM

Yeah, but that doesn't mean that you are safe if you only go up to 150psi or anything close to that. If you go high enough in pressure, the tank will weaken and weaken with every fill until it eventually bursts.
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#35 Doom

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Posted 23 March 2009 - 09:33 AM

Most people in this thread are dangerously ignorant of the issues here.

Not all PET bottles are the same. Bottles for non-carbonated beverages such as water often are not designed to hold pressure, even if they appear to be otherwise like carbonated bottles. There also are many differences in bottles between brands. One brand of 2 liter bottle may burst at 100 psi and another may burst at 250 psi. Some bottles are lemons. And the bottles might not stand up to repeated pressurization as they are designed to hold pressure for a certain amount of time.

Unless I did a hydrostatic test on a bottle I would not use it beyond about 80 psi because I've read of bottles bursting at pressures that low and even lower. A hydrostatic test fills the pressure vessel with water and applies pressure until a certain level to ensure the pressure vessel is safe at that level. Water won't store much energy so this is a very safe procedure.

Making the PET bottle fail safe is the idea if you are going to use a PET bottle. That's where wrapping the bottle in duct tape and such would fail. If you leave a weak spot, that'll burst and leak from there (or so it would in theory--practice often is unpredictable). That weak spot would make it "fail safe", or at least relatively safe. Wrapping the bottle in denim or some other fabric can help but won't necessarily will.

A "safety factor", that is, the number of times below the failure stress the operating stress is, is very important. Getting useful gas mass from a PET bottle requires pressures near the burst pressure. The general safety factor over burst pressure for pressure vessels is 2.5. So if a pressure vessel bursts at 150 psi, you should operate at a maximum of 60 psi to satisfy the safety factor. As the burst pressures vary from brand to brand and even bottle to bottle within brands, no specific recommendation can be made, but I think 60 psi should be safe for most bottles.

Temperature is also something to consider. Pumping gas increases it's temperature and extremely hot temperatures are easily possible (this is called adiabatic heating). Temperatures of 600+ degrees Fahrenheit are not uncommon if you neglect heat transfer in your analysis. The high temperatures will be lost relatively quickly to heat transfer. The melting point of PET is 500 degrees Fahrenheit... of course, the plastic won't reach that temperature due to the specific heat of the plastic, but it certainly will be weakened as the heat is transfered. This is an issue soda bottle manufacturers don't worry about because they are not pumping gas into the bottle.

Even operating at safe pressures, I'd be afraid of sudden stresses on the bottle. If you fall on your bottle, will it burst? You could do some tests to figure it out. This is something the water rocket people, who typically pressurize 2 liters, don't have to worry about but I think is a very real possibility for Nerf. Spud gunners don't have to worry about this either as they are not playing a game with their creations.

I could keep going on, but I hope most get the point.

As for PVC LPA tanks, they're a disaster too. They can handle higher pressures, but their failure characteristics are far worse. At high pressure PVC shrapnel will not be contained by anything short of kevlar from what I've read. PVC pipe and fittings are also far more expensive and heavy than other options. Edit: I have made myself unclear on this. If you can not get a pressure vessel designed for gas pressure (of any material--as long as it's DESIGNED to handle gas pressure), PVC would be a better choice than PET bottles, but don't take this as a recommendation. It's still a bad idea.

I'd avoid these issues completely by using something designed to hold gas pressure like an aluminum LPA tank. Sadly the only people who seem to have noticed the existence of such tanks are Draconis, myself, and a few others I've suggested them to.

Edited by Doom, 23 March 2009 - 06:53 PM.

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#36 VelveetaAvenger

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Posted 23 March 2009 - 10:19 AM

I wonder why everybody is complaining about pop bottles in this thread, but not this one. He's been charging his to 100 psi with no problems so far.


Can do.

Edited by VelveetaAvenger, 23 March 2009 - 10:37 AM.

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#37 VACC

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Posted 23 March 2009 - 10:26 AM

I wonder why everybody is complaining about pop bottles in this thread, but not this one. He's been charging his to 100 psi with no problems so far.

Can Do


Thank you for your edit. I just don't want us to dismiss any safety concerns. If anyone has some hard data on the subject feel free to share it. However, in the absense of reliable data, airing on the side of safety is something we're going to try and encourage.

On the subject of the other thread, you are correct. I am going to make a mention of this policy in there. Thanks.
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#38 CaliforniaPants

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Posted 24 March 2009 - 09:19 PM

So are we saying the a PVC tank like CS made would NOT be advised to use in this situation? Or would it be advised to use it how CS said, with a pressure gauge and watching how high you take it?
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#39 Doom

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Posted 24 March 2009 - 09:52 PM

This is what I said in summary:

aluminum LPA tank >> PVC LPA tank > PET bottle

PVC is definitely better than PET, but use some safety features like a pop-safety valve (available from McMaster-Carr and in some home improvement or hardware stores). Use a safety factor over the already conservative pressure rating. Do not use the tank in cold or very hot weather. Be gentle with the tank. Check it periodically for problems. Do all of that and your PVC tank should be fine, but be aware that plastic is rarely used for gas pressure because of the potential shrapnel problem.

Smaller diameters of PVC pipe in other components have higher pressure ratings and are consequently that much safer, but I still would suggest avoiding them and using metal pipe for pneumatics. Metal pipe is not expensive except in large sizes, so these price restrictions are actually good because they prevent obscenely large gas chambers.

If you have an aluminum LPA tank (and they're really easy to get and cheap), then you don't worry about these things. I'm so impressed with my aluminum tank I'm wondering why I hadn't heard about them before I considered buying one.

Edited by Doom, 24 March 2009 - 09:55 PM.

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#40 CaliforniaPants

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Posted 24 March 2009 - 10:02 PM

This is what I said in summary:

aluminum LPA tank >> PVC LPA tank > PET bottle

PVC is definitely better than PET, but use some safety features like a pop-safety valve (available from McMaster-Carr and in some home improvement or hardware stores). Use a safety factor over the already conservative pressure rating. Do not use the tank in cold or very hot weather. Be gentle with the tank. Check it periodically for problems. Do all of that and your PVC tank should be fine, but be aware that plastic is rarely used for gas pressure because of the potential shrapnel problem.

Smaller diameters of PVC pipe in other components have higher pressure ratings and are consequently that much safer, but I still would suggest avoiding them and using metal pipe for pneumatics. Metal pipe is not expensive except in large sizes, so these price restrictions are actually good because they prevent obscenely large gas chambers.

If you have an aluminum LPA tank (and they're really easy to get and cheap), then you don't worry about these things. I'm so impressed with my aluminum tank I'm wondering why I hadn't heard about them before I considered buying one.


Okay, thanks for clearing that up. Makes things much clearer.
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